1. Field of the Invention
This invention pertains generally to novel proton conducting membranes (PCMs) and the components utilized to produce these PCMs. More particularly, the subject invention relates to novel PCMs and their constituent components comprising hydrogen cyano fullerenes (HC60(CN)x as a proton-source agent and often poly(ethylene oxide) attached fullerenes (C60(PEO)y) as mixing agents to facilitate PCM formation with a host polymer.
2. Description of Related Art
The subject invention is utilized as a major component of a polymer electrolyte fuel cell (PEFC). PEFCs are generally comprised of three major components: the anode; the proton conducting membrane (PCM, the subject invention area); and the cathode. The PCM plays a critical role of transporting a proton from the anode to the cathode. It has to be highly proton conductive and also mechanically, thermally, and chemically stable. Water is produced at the interface between the cathode and the membrane. This water can be problematic, as discussed below, in operation of a PEFC. Lack of suitable membrane availability has been hindering the commercialization of PEFC. Water management is one of the most difficult issues in operating a PEFC. The water in the PEFC is produced as a product at the cathode side in PEFC. A breakdown in water balance between production and loss of water at the cathode side often results in water flood, while the anode interface with the membrane may suffer from water depletion due to water transportation toward the cathode side. Both the flood and the depletion may increase the cell over-potential which results in loss of power. Furthermore, the most commonly used PCMs are based on sulfonated perfluoropolymers that need to be fully humidified to be functional during the operation of the PEFC. Thus, these sulfonated perfluoropolymers not only require a humidifier, but also need an even distribution of water across the membrane which becomes an additional concern because of the membrane's high dependence on water.
Dry operation of PEFC may alleviate some of the water management problems. In fact, there is a strong demand in the auto industry as well as the distributed power generation industry for PEFC functional under low relative humidity (RH) (<50% RH). [Mathias, M.; Gasteiger, H.; Makharia, R.; Kocha, S.; Fuller, T.; Xie, T.; Pisco, J. Preprints of Symposia—American Chemical Society, Division of Fuel Chemistry 2004, 49(2), 471-474] Currently, no commercially available PCM meets this demand. NAFION, the industrial standard PCM by DuPont, is widely used in PEFC; yet it is sensitive to humidity, a very undesirable characteristic. Other existing proton conducting membranes, commercially available or under development, are as good or even better than NAFION under fully humidified condition, but very few outperform NAFION under low humidity conditions.
One existing PCM is disulfonated poly(arylene ether sulfone) copolymer (BPSH) developed by McGrath and coworkers. [Wang, F.; Hickner, M.; Kim, Y. S.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2002, 197, 231] Though BPSH is thermally stable and mechanically durable, and widely used as one of the most advanced alternative PCM, its proton conductivity under low RH (<80%) is lower than that of NAFION. Lack of membranes capable of functioning under low RH, (i.e., maintaining high conductivity, ˜10−1 S cm−1) has been an obstacle to bringing PEFC to market. The challenge for the industry is how to improve the conductivity of PCMs, where water plays a vital role in proton transportation, under dry condition.
A typical approach previously attempted to improve the conductivity of PCMs under low RH has been to increase the degree of sulfonation in the PCM in an attempt to increase the overall conductivity. [Tchatchoua, C.; Harrison, W.; Einsla, B.; Sankir, M.; Kim, Y. S.; Pivovar, B.; McGrath, J. E., Preprints of Symposia—Am. Chem. Soc., Div. of Fuel Chem. 2004, 49(2), 601] The problem with such an approach is that the membrane tends to swell more with a higher degree of sulfonation, which is detrimental in operation of fuel cell since the dimensional stability of the PCM is a key to the operation. Also, there is synthetic difficulty associated with increasing degree of sulfonation. Furthermore, there is a theoretical limit to the conductivity due to the sulfonyl groups (—SO3H) in the membrane.
An existing alternative approach to improve proton conductivity is a fabrication of composite membranes based on the conventional water-based PEM and inorganic/organic additives including SiO2 and heteropolyacids (HPA). [Shao, Z-G.; Joghee, P.; Hsing, I-M. J. Membr. Sci. 2004, 229, 43] Especially, HPA has been widely used to improve the performance of proton conducting membranes. [Herring, A. M.; Turner, J. A.; Dec, S. F.; Sweikart, M. A.; Malers, J. L.; Meng, F.; Pern, J.; Horan, J.; Vernon, D. Abst. 228th Am. Chem. Soc. National Meeting, Philadelphia, Pa., Aug. 22-26, 2004 FUEL-0053] The problems with HPA, however, are that it is water-soluble, thus leaches out, and the proton conductivity is sensitive to humidity. [Katsoulis, D. E. Chem. Rev. 1998, 98, 359] Hence, immobilization of HPA in a membrane is a particularly important issue. [Kim, Y. S.; Wang, F.; Hickner, M.; Zawodzinski, T. A.; McGrath, J. E. J. Membr. Sci. 2003, 212, 263]
An existing and more radical approach to improve proton conductivity is to replace water altogether. PCM with low volatile solvents such as imidazole have been utilized to replace water. [Kreuer, K. D.; Fuchs, A.; Ise, M.; Spaeth, Maier, M. J. Electrochim. Acta 1998, 43, 1281] Though the proton conductivity of 10−2 S cm−1 has been achieved at high temperatures, imidazole is known to poison the Pt catalyst and also is subject to diffusing out of the membrane, which is currently fixed through chemical attachment to a host polymer. [Schuster, M. F. H.; Meyer, W. H.; Schuster, M.; Kreuer, K. D. Chem. Mater. 2004, 16, 329.] Also, work exists in which a polybenzimidazole membrane was doped by H3PO4 (PBI/H3PO4). [Fontanella, J. J.; Wintersgill, M. C.; Wainright, J. S.; Savinell, R. F.; Litt, M. Electrochimica Acta 1998, 43, 1289.] Yet, H3PO4 is known to be leached out by water on the cathode side. Improvement of the performance of a PBI/H3PO4 membrane has been achieved through the use of polyphosphoric acid, however, the poor performance at low temperature and leaching out of H3PO4 by water condensation remain unsolved. [Zhang, H.; Chen, R.; Ramanathan, L. S.; Scanlon, E.; Xiao, L.; Choe, E-W.; Benicewicz, B. C. Prep. Div. Fuel Cehm. Am. Chem. Soc., Philadelphia, Pa., Aug. 22-26, 2004, 49, 588.] In another approach to replace water, inorganic solid acids such as CsHSO4 have been used. [Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Nature (London, United Kingdom) 2001, 410, 910.] However, there are concerns regarding this solid acid: reduction of the sulfur in the CsHSO4 electrolyte may occur over time, the reaction with hydrogen forms hydrogen sulfide, and also a poisoning to the Pt catalyst may occur. Other solid acids may be less problematic, but the stability of the materials remain problematic since the operation temperatures for these solid acids are close to their thermal decomposition temperatures. Thus, anhydrous (non-water) membranes have not reached a practical stage for operation of PEFC.
Although limited details are provided, a journal article by Saab et al. provides the first limited experimental data on the ionic conductivity of chemically functionalized fullerene. [Saab, A. P.; Stucky, G. D.; Passerini, S.; Smyrl, W, H, Fullerene Science and Technology, 1998, 6, 227.]
U.S. Pat. No. 6,495,290 B1 discloses proton conducting materials composed of carbon materials including fullerenes with functional groups attached to them. [Hinokuma, K., Ata, M., J. Electrochem. Soc. 150 (2003) A112.] It is claimed that the '290 materials can be used for PCM under dry condition. The best conductivity achieved using their materials under dry condition was 10−4 S cm−1, not very high for operation of a PEFC. The difference from the current subject invention is that: (i) the subject invention's performance is much higher, ˜10−2 S cm−1, than theirs, though the subject invention PCM also uses different fullerene-based materials; (ii) their materials lose performance as the content of their fullerenes in the PCM decreases below 80 wt %, while the subject invention PCM exhibits high performances with only 20 wt % of the subject novel fullerenes in a host polymer; and (iii) the subject invention functional groups attached to the fullerenes are completely different from those listed, suggested, or taught in '290. Furthermore, the '290 approach is to use fullerene as a carrier of proton hopping sites such as the OH groups for proton transportation where a proton is transported between the functional groups attached to fullerene. On the contrary, the subject invention uses novel fullerene derivatives as strong proton sources, i.e., the function in the subject invention is different from '290. Thus, a difference is that the '290 invention relies on the functional groups on fullerenes for proton transportation, while the subject invention uses water as the proton transportation medium and the derivatized fullerenes promote proton conduction as a proton-source, especially under low humidity. Additionally, when cyano groups (—CNs) are mentioned in '290 the cyano groups are considered to be only “electron attractive groups” that may be “introduced together with” the other listed critical functional groups and only serve to assist the non-cyano functional groups that must be present too.